Stem cell populations have been identified in many tissues and are thought to constitute a source of tissue renewal in quiescent, regenerative and pathological conditions. Tumor stem cells are the cell renewal source of a neoplasm and also serve as the seeds of metastatic spread of cancer. While rapidly proliferating tissues such as bone marrow, gut, and epidermis are known to be organized into stem cells and lineages of maturing descendants, the evidence for parallel phenomena in other tissues has been debatable.
Stem cell markers are useful to diagnose cancers and to treat pathological conditions characterized by abnormal or insufficient function of differentiated cells of a mature organ. However, stem cells have been difficult to identify and isolate.